The present invention relates to the field of television signal processing, and in particular to a method of scaling an alphanumeric character, such as a teletext character.
In order to display information on a matrix-like-constructed display device (e.g., a display screen) a known technique is to employ characters that are constructed in a matrix-like manner. An example of such characters involves teletext characters that function to display teletext information transmitted during the blanking interval in television transmission. These teletext characters comprise, irrespective of the language in which the text display is implemented, the characters that are required to display the textual and numerical information on the display device, (i.e., the numerical characters of the specific language and the letters of the specific alphabet).
For teletext display in Europe, the individual teletext characters comprise a pixel matrix of 10×12 pixels (i.e., a matrix of 10 pixel lines and 12 pixel columns). The lines and columns represent the specific character units that cannot be further divided in the direction of the line or in the direction of the column. A teletext character of this type is illustrated in FIG. 1 for a character representing the letter “A.”
A standard teletext page comprises 25 lines of 56 characters each of the type shown in FIG. 1, such that a display device having 250 pixel lines and 672 pixel columns (250×672=25·10×56·12) is required to display this teletext page. Modern displays such as for example LCD displays, plasma displays, or Digital Light Processing (DLP) displays have resolutions that far exceed the resolution required to display a teletext page. Display of a teletext page on such a display device necessitates scaling of the individual characters in order to enable the teletext page to at least approximately fill out the available image raster.
This type of scaling using a scaling factor greater than one is comparatively easy to implement if the goal is scaling using an integer scaling factor. If, for example, instead of 250 lines there are (k·250) lines available for the display, where k is an integer, then it is sufficient simply to repeat k times the individual lines of the teletext characters for the teletext page, (i.e, to reproduce it in k successive lines of the image). A corresponding integer scaling can be achieved in the column direction by repeated reproduction of the individual columns of the columns forming the teletext page. Scaling in the case of non-integer scaling factors is more difficult, as is explained below based on an example:
On LCD, plasma, and DLP display devices, the resolutions commonly found are in the so-called WXGA format comprising 1280 image columns and 720 image lines (1280×720). To achieve “line-filling” display of a teletext character on this type of display device, scaling in the line direction is required using a non-integer scaling factor of 2.88 (=720/250). A compromise might be to round down the scaling factor to the next-lowest integer value of two (2), which measure would result in incomplete utilization of the available display area; 220 (=720−2·250) out of the total of 720 lines remain unused. Rounding up the scaling factor to the next-highest integer would result in one or a few lines not being displayed at the upper or lower margins of the teletext image, which is also unacceptable.
Another known approach for scaling individual characters by a non-integer scaling factor is to scale the individual lines differently based on a predetermined scheme. For example, an overall scaling factor of 2.8 can be achieved by scaling every fifth line by a scaling factor of two, while scaling the remaining lines by a scaling factor of three (3). However, this type of approach can result in asymmetries in the enlarged characters.
If the goal is to scale a character using a scaling factor less than one, then certain pixel lines or pixel columns must be deleted as a function of the scaling direction. This deletion of lines or columns is perceived by a viewer as being less objectionable if lines or columns of the character are deleted that are spaced at regular intervals, or if after deletion only those lines or columns of the original image remain which are spaced at regular intervals.
There is a need for a technique for scaling a character which comprises a number of character units that are indivisible in at least either a horizontal direction or a vertical direction wherein asymmetries in the display of the scaled character are avoided.